Operating conditions
R22
bar (g)
R407C
bar (g)
R134a
bar (g)
R404A/R507A
bar (g)
Working pressure range high side
10.9 - 27.7
10.5 - 29.1
6.7 - 20.2
12.7 - 31.1
Working pressure range low side
1.4 - 6.9
1.1 - 6.4
0.6 - 3.9
2 - 7.3
Maximum high pressure safety switch setting
28
29.5
20.5
31.5
Minimum low pressure safety switch setting *
0.5
0.5
0.5
0.5
Minimum low pressure pump-down switch setting **
1.3
1.0
0.5
1.8
*LP safety switch shall never be bypassed and shall have no time delay.
** Recommended pump-down switch settings: 1.5 bar (R22, R407C, R404A) or 1 bar (R134a) below nominal
evaporating pressure.
A low pressure (LP) safety switch
must be used.
Deep vacuum operations of a scroll compressor
can cause internal electrical arcing and scroll
instability. Danfoss scroll compressors exhibit
high volumetric efficiency and may draw very
low vacuum levels, which could induce such
a problem. The minimum low-pressure safety
switch (loss of charge safety switch) setting is
given in the following table. For systems without
pump-down, the LP safety switch must either be
a manual lockout device or an automatic switch
wired into an electrical lockout circuit. The LP
switch tolerance must not allow for vacuum
operations of the compressor.
LP switch settings
for pump-down cycles
with automatic reset are
also listed in the table below.
Low pressure
The SY/SZ240 to SY/SZ380 incorporate an
internal relief valve set to open between the
internal high and low pressure sides of the
compressor when the pressure differential
between the discharge and suction pressures
surpasses 31 to 38 bar.
This safety feature prevents the compressor from
developing dangerously high pressures should
the high pressure cutout, for whatever reason, fail
to shut down the compressor.
HP
LP
Relief valve
Internal pressure relief valve
Cycle rate limit
Danfoss recommends a restart delay timer to
limit compressor cycling. The timer prevents
reverse compressor rotation, which may occur
during brief power interruptions.
The system must be designed in a way that
guarantees a minimum compressor running
time of 2 minutes so as to provide for sufficient
motor cooling after start-up along with proper oil
return. Note that the oil return may vary since it
depends upon system design.
There must be no more than 12 starts per
hour (6 when a resistor soft-start accessory is
introduced); a number higher than 12 reduces
the service life of the motor-compressor unit. If
necessary, place an anti-short-cycle timer in the
control circuit, connected as shown in the wiring
diagram section “Suggested wiring diagrams
logic”. A three-minute (180-sec) time out is
recommended.
Please contact Danfoss Technical Support for any
deviation from this guidelines.
Note that these two different low pressure
switches also require different settings. The low
pressure pump down switch setting must always
be within the operating envelope, for example
1.3 bar for R22. The compressor can be operated
full time under such condition. The minimum low
pressure safety switch setting may be outside the
normal operating envelope and should only be
reached in exceptional (emergency) situations,
for example 0.5 bar for R22.
T
T
TH
TH
KA
A1
A2
A3
A1
180 s
A2
KA
~
~
31
FRCC.PC.003.B4.02
Application Guidelines